Substitute natural gas may be prepared by a coal gasification process. A general discussion of coal gasification is set forth in Environmental Science and Technology, December 1971, Vol. 5, No. 12 in an article entitled "Gas from Coal, Fuel of the Future" by G. Alex Mills. Generally in the methanation portion of a coal gasification process, nickel is used as a catalyst to convert the synthesis gas, CO and H.sub.2, produced in the gasification step into methane as the final product.
The prior art teaches the placement of the nickel catalyst on an aluminum oxide substrate. When nickel is used on an aluminum oxide substrate, it is necessary to remove substantially all of the sulfur contained in the feed gas to the methanator. In particular, it is necessary to reduce the sulfur content of the feed gas to a level of less than about 0.1 ppm because the sulfur poisons the catalyst if present in any higher concentration. In order to achieve such a low sulfur content in the feed gas, it is necessary to clean initially the feed gas in a hot carbonate scrubbing operation or any other of a number of cleaning operations which reduces the sulfur to a level in the range of from 10 to 100 ppm. A second cleaning is necessary to reduce the sulfur concentration of the gas below 0.1 ppm. This second step is generally accomplished by an absorption process, whereby the sulfur is absorbed in zinc oxide, iron oxide or a number of other materials.
Sulfur is found in the feed gas to the methanator because sulfur is found in most coal supplies. On the average, the synthesis gas produced in a coal gasification process will contain at least 3,000 ppm sulfur prior to any sulfur removal treatment. The treatment of this gas to remove sulfur is a costly operation and when the treatment entails a two step removal process, the cost is increased. The process step of removing the sulfur from 10 to 100 ppm down to less than 0.1 ppm is very costly because of the large volume of gas which is processed to remove small amounts of sulfur and the expense of replacement or recovery of the absorbent.
It is an object of this invention to provide a methanation catalyst which is not easily poisoned by sulfur. It is a further object of this invention to provide a methanation process in which the gas fed to the process can contain in the range from 10 to 100 ppm of sulfur and can contain substantially higher sulfur levels for short periods of time.